1. Field of the Invention
This invention relates to a process for dressing kaolinite by flotation and more particularly to a process by which kaolin and feldspar can be separated in the ultra-fine particle range and may both be obtained in high yields of pure mineral.
2. Discussion of Related Art
Kaolinite is a naturally occurring industrial mineral with a large and growing demand for its many and varied applications as a filler in the paper and ceramics industry, in the plastics industry and in the manufacture of paints, lacquers, rubber and cables. Kaolinite is formed from feldspar by exogenous (weathering, ground water) and endogenous (hydrothermal hot solutions, underground gases) influences at predominately acidic pH values (Ullmanns Enzyklopadie der technischen Chemie. Vol. 13, page 509 (1977)). High quality demands on the kaolinite product in terms of purity are more difficult to satisfy, the higher the proportions of as yet non-kaolinite feldspar and quartz in existing and newly opened kaolinite deposits. Kaolinite has to be selectively separated from these accompanying minerals.
The dressing of the crude kaolin which, in addition to the main mineral kaolinite, generally contains feldspar, quartz and also various ferrous and titanium minerals is mostly carried out by wet processes in which the kaolinite-containing crude ore is suspended in water. The separation process is based on separation of the various mineral components according to particle size and specific gravity. Because the minerals quartz and feldspar, which accompany the kaolinite, are generally coarser ("anti-parallel particle distribution"), it is possible in this way to satisfactorily separate the coarser quartz and feldspar from kaolinite up to particle sizes of 2 .mu.m (cf. M. Clement and H. M. Troendle; Erzmetall 22, No. 3, 131 (1969)).
Because feldspar is also a raw material widely used in the ceramic industry, every attempt is made when separating kaolinite and feldspar from crude kaolin to obtain not only pure kaolinite, but also a feldspar product which satisfies the stringent demands for industrial utilization in the glass and ceramics industry. It is known that mechanical separation processes in aqueous pulp can be used for this purpose. However, the effectiveness of separation processes such as these reaches a limit when the particle sizes of kaolinite and feldspar in the ultra-fine particle range lie very close to one another, because the specific gravity of the two minerals (approx. 2.58 g/cm.sup.3) is substantially the same. In such a case, the demand for high purity of both end products, kaolinite and feldspar, is accompanied by an economically significant loss in the non-separable particle classes (ultra-fine particle range). The separation of kaolinite and feldspar in aqueous pulp is described in detail in B. M. Coope, Industrial Minerals, 1979, 33 to 49 and H. H. Murray, Int. J. of Mineral Processing 7, 263 (1980).
Flotation processes are used in the cleaning of minerals to remove heavy metal oxides, for example oxides of iron and titanium, from kaolinite and hence to improve the whiteness of the product. Separate processes for separating kaolinite from quartz on the one hand and feldspar from quartz on the other hand by flotation of the minerals in the presence of an amine as collector are also known from H. M. Troendle, M. Clement and B. Becher, Interceram 19, 185 and 268 (1970) corresponding to Chemical Abstracts 74, 102589 u (1972). Hydrochlorides and hydroacetates of long-chain aliphatic amines are used as collectors in acidic pulps.
Flotation tests with kaolin-quartz and feldspar-quartz mixtures are also described in H. M. Troendle, M. Clement and B. Brehler, Keramische Zeitschrift 21, 423 and 489 (1969) corresponding to Chemical Abstracts 72, 102251 m (1970). Aqueous pulps containing hydrofluoric acid and hydrochloric acid are used with long-chain aliphatic amines as collectors for separating the particular kaolinite and feldspar minerals from quartz. In the process where kaolinite is separated from quartz, it was also found that a significant reduction in the recovery of kaolinite at alkaline pH-values of the aqueous pulp is attributable to the fact that the non-dissociated amine molecules formed in accordance with equation (1) below are no longer adsorbed onto the negatively charged kaolin surface and hence prevent the kaolinite content of the pulp from being extracted with the ammonium salt: EQU R--NH.sub.3.sup.+ +OH.sup.- .revreaction.R--NH.sub.2 +H.sub.2 O (I)
The ultrafine range of kaolinite and feldspar particles is of particular interest for the dressing of aqueous pulps containing both minerals. Hitherto, separation of the two minerals in this particle range has not been possible on an industrial scale. However, it is this range which is particularly important in practice insofar as mineral mixtures having a particle distribution of 90% smaller than 30 .mu.m for kaolinite and 10% smaller than 30 .mu.m for feldspar accumulate during the washing out of the kaolinite-and feldspar-containing deposits. Kaolins suitable for use in the ceramics field have an ultra-fine particle fraction (smaller than 2 .mu.m) of 50% or more. In this particle size range, selective flotation in aqueous pulp presents considerable problems. Direct flotation for separating kaolinite and feldspar in this particle range has never been described before.
It is known that polyvalent cations are adsorbed on the surfaces of the mineral particles suspended in the aqueous pulp and are capable of influencing the floatability of these mineral particles within wide limits (B. Dobias, 6th International Congress on Surface-Active Agents, Zurich 1972, page 563 (1973)). In addition, polyvalent cations are capable of reacting with the collector surfactant to form complex compounds or sparingly soluble deposits and thus removing the surfactant from the desired adsorption process on the surface of the mineral particles. Experience has shown that this reduces the flotation yield unless increased quantities of the collector surfactant are used.
Depending on the composition of the system to be dressed by flotation, the same polyvalent cation can have both an activating effect and a deactivating effect for the chosen collector (surfactant) in the flotation of mineral particles. The question of which of the two properties dominates can only be empirically determined. In general, the flotation process itself is disturbed rather than promoted by these effects, of which the action mechanisms are not yet completely known in detail. Accordingly, suppressing the undesirable effect of polyvalent cations in the floation process is a special problem in the dressing of the particular minerals.
The activating or deactivating ("depressing") effect of polyvalent cations on silicate-containing minerals is known in part from the literature. Thus, it is reported in Chemical Abstracts 68, 116 041A (1968) that aluminium, iron and calcium ions have an activating effect in the flotation of quartz, feldspar and sillimanite. If the concentration of iron(III) and aluminium(III) salts is increased to values above 300 mg per liter, there is a distinct reduction in the activating effect at acidic and alkaline pH values in the flotation of silicates.
The use of cationic collectors in acidic media in the separation of quartz-feldspar sands by floation in the presence of copper(II), calcium, iron(III) and aluminium(III) ions is described in Chemical Abstracts 71, 5014h (1969). In strong sulfuric acid or hydrochloric acid media, the cations mentioned have an activating effect on feldspar and a depressing effect on quartz.
The separation of feldspar from accompanying materials by flotation in the presence of aliphatic amines or petroleum sulfonates in strong sulfuric acid solutions is described in Chemical Abstracts 79, 147 840p (1973). SiO.sub.2, Al.sub.2 O.sub.3 and Fe.sub.2 O.sub.3 are added as activitors to the aqueous pulps; the recovery of feldspar from the pulps increasing with increasing concentrations of collectors.
In Neue Bergbautechnik 9, 349 (1979), corresponding to Chemical Abstracts 92, 150,570 (1980), it is reported that, in the aqueous flotation of feldspar-quartz mixtures in the presence of long-chain aliphatic amines, aluminium trichloride has an activating effect on feldspar and a depressing effect on quartz in the pulp. However, the problem of separating kaolinite and feldspar in aqueous pulps is not discussed in any of the literature references mentioned above.